This invention relates to cardiac diagnostic and chronic therapeutic leads, and more particularly to fixation leads in which an electrode includes an anchoring element.
The utility of cardiac pacing leads is well recognized, both for carrying pulse stimulation signals to the heart from a pacemaker, and for monitoring heart electrical activity from outside the body. Many such leads are sufficiently flexible and small in diameter for intravenous introduction to a cardiac cavity, whereupon an electrode at the distal end of the lead is implanted into the endocardium to secure the lead. For this purpose, helical coils, barbs and other anchoring elements are provided, typically as part of the electrode.
The anchoring element must be sufficiently sharp to penetrate the endocardium and secure the electrode against becoming detached, for example due to contractions of the myocardium. During a critical period immediately after implant and prior to full fibrotic growth, usually three to twelve weeks, the anchoring element must provide substantially the entire force maintaining the electrode in its selected location. Given these requirements, it is not surprising that an effective anchoring element can become entangled in the vein, heart valve or other tissue encountered during its intravenous insertion.
The problem has given rise to numerous proposed solutions. For example, U.S. Pat. No. 3,974,834 to Kane granted Aug. 17, 1976 shows a sleeve which shrouds the sharp tip of a fixation helix, but collapses in accordion-like fashion as the helix is turned into the endocardium. In U.S. Pat. No. 4,282,885 to Bisping granted Aug. 11, 1981, a protective core is surrounded by the helix, and is movable axially relative to the helix. A wire, attached to the core, extends through the lead and can be pulled after lead insertion to withdraw the core, exposing the helix. U.S. Pat. No. 4,146,036 to Dutcher et al granted Mar. 27, 1975 discloses an extensible and retractable core surrounded by the helix.
Other solutions involve making the fixation element movable. For example, in U.S. Pat. No. 4,180,080 to Murphy granted Dec. 25, 1979, a spiral coil, normally recessed within a guide tube, can be rotated whereby it emerges beyond the tube. U.S. Pat. No. 3,844,292 to Bolduc granted Oct. 29, 1974 discloses a plunger outside of the body which, after release of two locking mechanisms, is movable to push outward a barb-like tip. A somewhat similar arrangement, involving a platinum piston movable to push a harpoon-shaped anchor beyond the end of a tubular electrode, is shown in U.S. Pat. No. 4,258,724 to Balat et al granted Mar. 31, 1981.
Such devices, while satisfactory in certain respects, are undesirable in that leads employing them must have a larger diameter. They often require additional tools, for example a stylet-type screw driver for rotating the helix. Further, such devices are often overly complex, diminishing their reliability and raising the possibility of a current leakage path between conductors of bipolar leads.
Therefore, it is an object of the present invention to provide a smooth, rounded covering for the anchoring element of a cardiac endocardial electrode to facilitate intravenous insertion of the electrode.
Another object of the invention is to provide such a covering which is soluble in body fluids, thereby to expose the anchoring element at a specified time after its initial insertion into the body.
Yet another object is to provide a simple, non-mechanical means for covering fixation mechanisms during intravenous insertion of a pacing electrode having an anchoring element, without requiring any longitudinal relative movement between the electrode and anchoring element.